The present invention relates, in general, to developer rollers used in electrophotography, and more particularly, to a roller and its process of manufacture having a surface with uniform mechanical and electrical properties.
In electrophotography (e.g., laser printers), the developer roller function is to develop a layer of toner on a photoconductor drum electrically discharged in an image pattern. A two layer, xe2x80x9ccoatedxe2x80x9d roller will develop a fixed quantity of toner per volt of development bias that is predetermined by the dielectric thicknesses of the photoconductor, the toner and the developer roller. This development characteristic is independent of process speed, within limits. In contrast, a solid roller of a single resistivity develops a quantity of toner based on the dielectric constants of the photoconductor and the toner, and the resistance of the roller in the photoconductor nip. This is dependent on the process speed. In addition, compared to a solid roller or a single resistivity, a two-layer roller has a longer time constant than a solid roll. A longer time constant material leaves a higher effective development surface potential on the developer roll at the entry to the photoconductor nip, thus improving the single pel dot print performance of the roller. Thus, two-layer rollers are thus generally preferred for electrophotographic developer rollers.
In U.S. Pat. No. 5,874,172, which is hereby incorporated by reference in its entirety, an economical and effective approach is disclosed of achieving a two-layer roller that is cast as a solid roller with the outer resistive layer achieved by oxidization. A cast urethane, electrically conductive rubber roller is produced with a surface layer of high electrical resistivity, mimicking the more difficult to produce coated rollers. The roller is composed of a polydiene, such as polyisoprene, and more specifically polybutadiene, either as a polyol or a urethane prepolymer, blended with a second polyurethane prepolymer and a conductive additive such as ferric chloride. The surface of the cured roller is oxidized by baking in air at a fairly high temperature (e.g., greater than 80 degrees C.) for several hours. The reaction of oxygen in the air with the polybutadiene, catalyzed by the ferric chloride, oxidizes the surface of the roller. In addition, an antioxidant material such as a hindered phenol is also added to the pre-cured urethane mixture to minimize further oxidization. Thereby, the service life of the roller is extended by at least a factor of ten.
For this and other urethane mixtures, the generally accepted approach to molding rollers is to use a horizontal, open six-cavity mold 10 such as depicted in FIG. 1. This orientation and being open along the length of the cavity provides air bubbles entrained into the pre-cured urethane mixture a relatively short vertical distance to travel. Ridding the roller of bubbles, even small ones, is necessary to achieve a uniform electrical characteristic for good printing performance. Bubbles at the surface create a particularly unacceptable defect, pitting the surface and causing toner to accumulate on the roller. The corresponding over-development at this location creates a corresponding dot on each printed page. Having a short distance for the bubbles to travel is necessary because the urethane mixture is viscous and becomes more viscous as it begins to cure.
Using an open-cavity has disadvantages during the cure cycle. Heating the mold is used to enhance and accelerate the curing of the urethane mixture; however, the open mold causes a variation in temperature across the urethane mixture and a corresponding variation in the mechanical and electrical characteristics of the resulting cured urethane. This is especially true of the top-most portion of the material that is destined to be removed.
After the initial curing process is completed, an operator manually removes the one solid part 12 consisting of six identical, D-shaped rollers connected together, each having a steel shaft 14. Manual separation, such as at 16, is required. Then, each separated D-shaped roller 18 is rough ground (xe2x80x9cG1xe2x80x9d), where an excess portion 20 of approximately 30% of the urethane is removed and scrapped. A benefit of the rough grinding is that the excess portion 20 is more prone to having bubbles and having been improperly cured. However, this G1 grinding step rapidly transforms the D-shaped roller 18 from a D-shape to a round shaped roller 22, as depicted in FIG. 2. The rapid grinding generates a considerable amount of heat at the surface of the roller 22 and mechanical stress between the shaft 14 and the urethane cylinder 24, which may detrimentally affect the functional characteristics of the roller 22. The rough ground roller 22 is then placed in an oven for post curing to achieve the final physical properties of the urethane. In addition, precision grinding and finishing steps are performed to achieve the desired geometry and surface finish.
It has long been believed that molding rollers in a round cross-section configuration would offer significant manufacturing cost advantages by eliminating the xe2x80x9cG1xe2x80x9d grinding step, reducing the amount of waste material, and providing more uniform properties around the roller. However, problems with bubbles, gate marks (i.e., entry point for injecting urethane into the mold cavity), parting lines, and knit lines, all of which can create defects on the printed page, were difficult to overcome.
Furthermore, prior attempts at a closed mold encountered the difficulties of shrinkage of the urethane during curing. In an open mold, the longitudinal ends may be held fixed to the desired length, with required material drawn in from the excess portion 20 as the urethane material shrinks. However, with the cylindrical cross section of a closed mold, the excess for shrinkage had to be drawn from the longitudinal ends. Thus, a secondary operation to trim the ends of the urethane roller is required to achieve the proper length, thus offsetting some manufacturing efficiency over the open mold.
Consequently, a significant need exists for an approach for molding urethane rollers suitable as electrophotographic developer rollers that have fewer material defects and simplified shaping and finishing steps.
The invention overcomes the above-noted and other deficiencies of the prior art by providing a method of using a closed cylindrical mold to produce electrophotographic developer rollers of the desired diameter without rough grinding and the desired length without trimming. A tube has a portion that is cylindrically shaped for the desired diameter and has sufficient length for the desired height of the roller. A shaft that will become part of the roller spaces a top end cap and a bottom end cap within the tube, each cap defining one end surface of the resulting roller. An injection gate through the tube into the cylindrical cavity is used to inject a urethane mixture that cures (hardens) to become roller, which is then precision ground, given finishing treatments, and oxidized in an oven. Thereby, the advantages of a solid roller are achieved without having to rough grind a non-cylindrically shaped roller.
In one aspect of the invention, prior to filling the cylindrical cavity of the tube with urethane mixture, the cylindrical cavity is filled with carbon dioxide, displacing the air. Then, when filling the cylindrical cavity with mixture, any bubble formed by the mixing of the carbon dioxide with the urethane mixture will rapidly be dissolved into the urethane mixture. Thereby, rollers are produced without the defect of bubbles that would degrade or preclude their use as electrophotographic developer rollers.
In another aspect of the invention, after filling the cylindrical cavity with the urethane mixture, the assembly of the top end cap, shaft, and bottom end cap, as well as the urethane mixture captured therein, is repositioned within the tube such that the injection gate is separated from the urethane mixture in the cylindrical cavity. Thus, no gate mark is formed in the urethane mixture as it cures, avoiding another defect that would degrade or preclude the use of the roller. Moreover, a secondary process is avoided of removing cured material in an injection gate prior to being able to demold the roller from the mold.
In yet another aspect of the invention, one of the end caps and the tube define a cavity into which excess urethane mixture flows through a small gap during injection of the urethane mixture into the cylindrical cavity. During curing, the urethane mixture contracts, drawing in some or all of the excess urethane mixture. Thereby, the desired length and contour of the ends of the roller are provided without trimming. In some applications, the gap is annular and at a diameter from the shaft greater than the final diameter after precision grinding, and is thus removed during this step without trimming of the ends.
These and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.